Author

Date of Graduation

Degree

Department

Committee Chair

Robert Pavlowsky

Keywords

water quality, reservoir drainage, sediment, phosphorus, nitrogen

Subject Categories

Hydrology | Sedimentology | Water Resource Management

Abstract

Construction and subsequent draining of reservoirs can have dramatic affects on the release of nutrients and sediments to waterways. This study describes how the temporary draining of a small reservoir for dam repair influences downstream water quality. The Valley Mill Reservoir has a surface area of 6.1 hectares and volume of 150,000 m3 when filled. Water chemistry monitoring and water sampling were performed at six sites during baseflow and ten sites during runoff events for one year. Water samples were analyzed for total nitrogen (TN), total phosphorous (TP), and total suspended sediment (TSS) concentrations. Results indicate that draining of Valley Mill Reservoir caused only minor changes in water chemistry. However, reservoir drainage caused significant erosion of the exposed lake bed as well as the stream channel upstream of the reservoir. Increases in TSS lagged behind drainage but increased dramatically once drainage was complete. Mean TSS increased from 7.5 mg/L upstream of the reservoir to 20.7 mg/L in reservoir outflow during baseflow. During storm events, TSS increased over 100 percent to nearly 100 mg/L in the drained reservoir outflow, with a maximum concentration of 525 mg/L. The increase in TSS resulted in TP increases during baseflow and storm events, since TP is known to attach to sediment. Mean outflowing TP increased by 10 percent to 43 ug/L during baseflow and by 20 percent to 207 ug/L following storm events. Total nitrogen remained below 5 mg/L at all monitoring sites and decreased by 5 to 15 percent after flowing over the exposed lakebed. Therefore, draining of Valley Mill Reservoir may have caused degradation of water quality downstream of reservoir outflow due to large amounts of sediment and phosphorus being released from the drained reservoir.